1. Field of the Invention
The present invention relates to a digital camera, and more particularly, to the improvement of a focusing function in the digital camera.
2. Description of the Related Art
A wide variety of interchangeable lenses have been available for lens-interchangeable cameras capable of changing photographing lenses since film cameras were mainstream. Those interchangeable lenses have been designed with the assumption that the AF (focus detection) mechanism provided integrally with the camera body would support a TTL phase-difference AF system (hereinafter referred to as “TTL phase-difference AF”).
Conventionally, a so-called imager AF system (hereinafter referred to as “imager AF”) has also been frequently employed as well as the TTL phase-difference AF. The imager AF is an AF system for calculating a focus evaluation value representing the acutance of an image from image data output from an image pickup device for capturing a subject image while scanning the photographing lens, i.e., while changing the position of a focusing lens in the photographing lens in an optical axis direction to determine a lens position at which the focus evaluation value becomes the maximum value. To be more specific, in the imager AF, the photographing lens is scanned to determine a lens position at which high frequency components of an image signal from the image pickup device take the maximum value. This imager AF is also called hill-climbing AF or contrast AF. The imager AF is the mainstream AF system for compact digital cameras and video camcorders.
Now that digital cameras are mainstream, it is desired that the interchangeable lenses can be used for the digital cameras. In case of existing lens-interchangeable cameras, for example, only for digital single-lens reflex cameras, the dominant AF system is the TTL phase-difference AF, and hence the interchangeable lenses are usable. However, even if the AF system of the camera body is the imager AF, it is still desired that the interchangeable lenses be usable. In this case, making the existing interchangeable lenses usable brings significant merits to users even though some restrictions are required for actual use.
There are conventionally known a wide variety of lens-interchangeable cameras employing the imager AF as their AF system. For example, all of cameras disclosed in Japanese Patent Application Laid-Open Nos. 06-6660, 06-181532, and 08-223469 are lens-interchangeable cameras employing the imager AF as their AF system.
However, if interchangeable lenses designed to support conventional TTL phase-difference AF are used for these lens-interchangeable cameras employing the imager AF as their AF system, typified by those disclosed in Japanese Patent Application Laid-Open Nos. 06-6660, 06-181532, and 08-223469, there arises a problem that a sufficient focusing speed cannot obtained. This problem will be described below.
In the TTL phase-difference AF, the photographing lens is moved by an amount of lens travel corresponding to a detected defocus amount of the photographing lens to achieve focusing of the lens. In other words, it is already known how far to move the photographing lens at the start of lens traveling. In this traveling of the lens, a DC (Direct Current) motor, an ultrasonic wave motor (USM), or the like is used as an actuator to move the focusing lens in the interchangeable lens.
On the other hand, in the imager AF, since it is not known how fat to move the photographing lens at the start of lens traveling, this system needs to seek a final destination point of traveling of the photographing lens while moving the lens. Thus, the imager AF involves repetition of fine traveling of the photographing lens at predetermined pulse intervals. Therefore, in the imager AF, a stepping motor is best suited as the actuator to move the focusing lens in the interchangeable lens. However, the existing interchangeable lenses are designed to support the conventional TTL phase-difference AF, i.e., they are not optimized to move the focusing lens in the imager AF.